Cytoplasmic lipid droplets as the possible eventual cellular fate of active forms of cyclosporin.

The activity of cyclosporins can be defined with regard to their ability to inhibit the proliferation of susceptible lymphoblastoid cell clones. All active cyclosporins cause the emergence of highly refringent globular bodies, independently of cell susceptibility to cyclosporin. Cyclosporins devoid of proliferation-inhibition capability do not cause such alterations in cell morphology. The use of dansylated active cyclosporins suggests that within a few hours of treatment in vitro, most if not all cyclosporin is contained within those globular bodies. By using cytochemical methods allowing differential staining, we show here that these cyclosporin-containing structures are neither normal mitochondria (shown by use of rhodamine 123) nor normal lysosomes (shown by use of acridine orange under stringent staining conditions) but most probably lipid droplets (shown by use of perylene and various dansylated hydrophobic probes).